The New Zealand River Environment Classification (REC) organises information about the physical characteristics of New Zealand's rivers. Individual river sections are mapped according to physical factors such as climate, source of flow for the river water, topography, and geology, and catchment land cover eg, forest, pasture or urban. Sections of river that have similar ecological characteristics can then be grouped together, no matter where they are.

This information is mapped for New Zealand's entire river network - over 425,000 kilometres of river. Different types of rivers respond differently to the pressures placed on them - the REC can be used to highlight the most appropriate management tools and approaches to reduce these pressures for each river type. Information from the classification is used to develop policy, assess the environment, and report on the quality of river water.

Stream order is the numerical position of a tributary or section of a river within the entire network. Headwater streams are assigned a stream order of 1. When two tributaries of the same stream order meet, the order increments by one for the next section downstream. However, if two sections meet where one section has higher order than the other, the next section downstream has the same order as the highest upstream section.